1. Field of the Invention
This invention relates to controlling variable resistance and more particularly to maintaining a constant impedance in variable capacitor arrays.
2. Description of the Related Art
In short-range wireless applications, the load of the transmitter is often high impedance as it consists only of the real portion of a high quality factor LC tank circuit. For these applications, a simple loop antenna on the PC board forms the inductor, L, and the transmitter usually incorporates at least a portion of the capacitive load, C. In order to tune out manufacturing variations in both L and C, it is desirable to make the transmitter capacitor variable so that the tank circuit can be tuned precisely to resonate at the desired operating frequency. That helps maximize the efficiency of the overall system by minimizing loss.
One way to produce a variable capacitance is to provide multiple capacitive elements, which can be selectively switched into the load under digital control. One problem with such an approach is that the switches have non-zero on resistance, and therefore cause a loss of power. Furthermore, the switch resistance, and therefore the amount of the power loss, depends on processing, temperature, and switching voltage. In many applications it is desirable for the transmitted power to be well controlled. Variation in switch resistance leading to variations in transmitted power can be somewhat alleviated by using large switch devices in the capacitor array. However, there are two main problems with that approach. One, the area of the capacitor array can become excessively large. Two, the parasitic capacitance associated with the switch causes the ratio of the “off” to “on” capacitance of the array to degrade. Ideally, the “off” capacitance would be zero; however, parasitic capacitance due to the capacitor itself and the switch cause the “off” capacitance to be larger than zero. A non-zero “off” capacitance reduces the tuning range of the variable capacitor because it reduces the difference between the capacitance of the array element when it is “on” versus when it is “off.” Even if the switch resistance is reduced by increasing the size of switch, this does not address the non-zero temperature dependence of the transmitted power.
Accordingly, it would be desirable to provide constant switch resistance in the capacitor array to avoid variation in transmitted power and to allow for improved “off” to “on” capacitance ratios.